The present invention relates to a switching circuit and a voltage measuring circuit, and more particularly, the invention relates to a switching circuit for connecting both ends of each of the batteries connected in series to a capacitor, and a voltage measuring circuit having the switching circuit.
There has been proposed, for example, a related switching circuit of the sort mentioned above as shown in FIG. 5 (e.g. JP-A-11-248755 and JP-A-2002-156392). As shown in FIG. 5, the switching circuit has one capacitor C with respect to a plurality of unit cells V1-Vn connected in series and a plurality of change-over switches S1-Sn+1 for sequentially connecting both ends of each of the unit cells V1-Vn to both ends of the capacitor C. In FIG. 5, each of the unit cells V1-Vn is formed with one battery.
The change-over switches S1-Sn+1 are provided such that (n+1) pieces of the switches are provided for n pieces of unit cells V1-Vn. More specifically, the plus side of the unit cell V1 and the minus side of the unit cell V2 connected to the plus side of the unit cell V1 are connected to the capacitor C via the common change-over switches S2. Further, there has been proposed another switching circuit such that as shown in FIG. 6 two of change-over switches S1-S2n are provided on both sides of each of the unit cells V1-Vn (e.g., JP-A-11-248757).
In the related switching circuits shown in FIGS. 5 and 6, relay switches have been employed as the change-over switches S1-Sn+1 and S1-S2n. However, the relay switch has a problem in respect of cost, size, durability and response speed; accordingly, there has developed a demand for use of semiconductor switches excellent in these respects as the change-over switches S1-Sn+1 and S1-S2n.
Nevertheless, because a parasitic diode is present across the source-drain of the semiconductor switch, an electric current may flow via the parasitic diode even when the change-over switches S1-Sn+1 and S1-S2n are turned off, so that it is difficult attempting to completely insulate the unit cells V1-Vn from the capacitor C.